1. Field of the Invention
The present invention relates generally to methods and apparatus for testing electrical circuits formed upon substrates such as but not limited to printed circuit substrates, ceramic substrates and semiconductor integrated circuit substrates. More particularly, the present invention relates to methods and apparatus for reliably testing electrical circuits formed upon substrates such as but not limited to printed circuit substrates, ceramic substrates and semiconductor integrated circuit substrates.
2. Description of the Related Art
Known in the arts of fabricating electrical circuits upon substrates such as but not limited to printed circuit substrates, ceramic substrates and semiconductor integrated circuit substrates is the use of electrical test apparatus, often in conjunction with the use of accelerated physical stress apparatus, to determine and/or assure the functionality and/or reliability of electrical circuits formed upon the substrates such as but not limited to the printed circuit substrates, the ceramic substrates and the semiconductor integrated circuit substrates. The use of electrical test apparatus, with or without the use of accelerated physical stress apparatus, may be employed at several process stages within electrical circuit fabrication, including but not limited to: (1) early electrical circuit fabrication process stages where the electrical test apparatus may be employed as an in-process test apparatus; and (2) later electrical circuit fabrication process stages where the electrical test apparatus may be employed as a final test apparatus and/or a product certification apparatus.
Electrical test apparatus employed in testing electrical circuits formed upon substrates such as but not limited to printed circuit substrates, ceramic substrates and semiconductor integrated circuit substrates typically provide direct physical contact to conductor contacts such as but not limited to bond pads which access upon the substrates electrical circuits whose electrical characteristics it is desired to test. The electrical test apparatus typically make physical contact with the conductor contacts through use of needle like tapered conductive probe tips which bridge from a probe card which forms a part of a tester head assembly within an electrical test apparatus to the conductor contacts which access the electrical circuits whose electrical characteristics it is desired to test. Conductive probe tips are typically configured within a probe card within a tester head assembly within an electrical test apparatus in a fashion such that they dissipate and distribute a loading pressure employed when contacting the conductive probe tips to the conductor contacts formed upon a substrate.
Shown in FIG. 1 and FIG. 2 is a pair of schematic cross-sectional diagrams illustrating operation of an electrical test apparatus conventional in the art of testing electrical circuits formed upon substrates such as but not limited to printed circuit substrates, ceramic substrates and semiconductor integrated circuit substrates. Shown in FIG. 1 is a platen 10 upon which is fixed a substrate 16 which may be chosen from the group of substrates including but not limited to printed circuit substrates, ceramic substrates and semiconductor integrated circuit substrates. Positioned in a first position adjoining but not in contact with the substrate 16 is a tester head assembly 12 which comprises a mounting block 122 having fixed thereupon a probe card 124 which in turn has disposed thereupon a minimum of one conductive probe tip 126 through which contact to a conductor contact (not shown) upon the substrate 16 is ultimately made. Also shown in FIG. 1 is an aperture 128 through the tester head assembly 12, which aperture 128 allows for access of a microscope 14 employed in assuring proper registration of the probe tips 126 with corresponding conductor contacts (not shown) formed upon the substrate 16. Shown in FIG. 2 is a schematic cross-sectional diagram illustrating the results of further processing of the substrate 16 within the electrical test apparatus as illustrated in FIG. 1. Shown in FIG. 2 is the results of repositioning the tester head assembly 12 from the first position with respect to the platen 10 as shown in FIG. 1 to a second position with respect to the platen 10 such that the conductive probe tips 126 contact the conductor contacts (not shown) within the substrate 16, thus allowing electrical measurements to be obtained of electrical circuits accessed through the conductor contacts (not shown) formed upon the substrate 10.
While the use within electrical test apparatus of conductive probe tips as contact elements bridging probe cards with conductor contacts formed upon substrates such as but not limited to printed circuit substrates, ceramic substrates and semiconductor integrated circuit substrates has become quite common in the art of testing electrical circuits formed upon substrates such as but not limited to printed circuit substrates, ceramic substrates and semiconductor integrated circuit substrates, the use within electrical test apparatus of conductive probe tips as contact elements bridging probe cards with conductor contacts formed upon substrates is not entirely without problems. In particular, it is known in the art that use of electrical test apparatus employing conductive probe tips as contact elements bridging probe cards with conductor contacts formed upon substrates suffers from several problems which relate to contact resistance increases between the conductive probe tips and the conductor contacts, including but not limited to: (1) conductive probe tip contact resistance increases due to conductive probe tip contact with adventitious non-conductive particulate deposited upon conductor contacts; (2) conductive probe tip contact resistance increases due to oxidation of conductor contact material which has transferred to a conductive probe tip; and (3) conductive probe tip contact resistance increases due to oxidation of conductive probe tip material when testing electrical circuits at elevated electrical test apparatus temperature. Such conductive probe tip contact resistance increases are undesirable since they may provide erroneous indications of non-functional or unreliable electrical circuits formed within substrates, thus erroneously reducing fabrication yields those substrates. It is thus towards providing: (1) an electrical test apparatus employing a conductive probe tip bridging to a conductor contact accessing an electrical circuit upon a substrate such as but not limited to a printed circuit substrate, a ceramic substrate or a semiconductor integrated circuit substrate; and (2) a method for employing the electrical test apparatus which employs the conductive probe tip bridging to the conductor contact accessing the electrical circuit upon the substrate, while avoiding the foregoing conductive probe tip contact resistance increases, that the present invention is directed.
The use of electrically conducting probes as a means for providing measurements within various technology applications, including but not limited to electrical circuit fabrication applications, is generally known within the various pertinent technology arts. For example, Gray et al., in U.S. Pat. No. 5,415,042 discloses a measuring probe for detecting the presence of an electrically conducting liquid or granular material contained within a vessel, The measuring probe employs a gas purge to clear the electrically conducting liquid or granular material from between the conducting electrodes of the measuring probe. In addition, Barnett et al., in U.S. Pat. No. 4,872,356 discloses a resistivity probe apparatus which provides an enclosure for a resistivity probe employed in monitoring resistivity of deionized water employed in rinsing semiconductor substrates employed within integrated circuit fabrication. The resistivity probe apparatus provides several advantages directed towards providing accurate and reproducible resistivity measurements from the resistivity probe housed within the resistivity probe apparatus.
Most pertinent to the present invention, however, is the disclosure of Takahashi et al. in U.S. Pat. No. 5,280,236. Takahashi et al. disclose an integrated circuit test apparatus comprising a probe card with conductive probe needles disposed on the probe card, where the conductive probe needles are formed of a cobalt based alloy comprising at least ten percent chromium. When properly polished or passivated, the conductive probe needles so formed are less susceptible to accumulating contaminants which increase contact resistance of the conductive probe needles when the conductive probe needles bridge to conductor contacts formed upon an integrated circuit substrate.
Desirable in the art are additional electrical test apparatus which employ conductive probe tips to bridge to conductor contacts which access electrical circuits formed upon substrates such as but not limited to printed circuit substrates, ceramic substrates and semiconductor integrated circuit substrates. Particularly desirable in the art are additional electrical test apparatus which employ conductive probe tips to bridge to conductor contacts which access electrical circuits formed upon substrates such as but not limited to printed circuit substrates, ceramic substrates and semiconductor integrated circuit substrates, while avoiding contact resistance increases of those conductive probe tips. It is towards providing an electrical test apparatus in accord with the foregoing goals, and a method for using the electrical test apparatus in accord with the foregoing goals, that the present invention is directed.
A first object of the present invention is to provide: (1) an electrical test apparatus which employs a conductive probe tip to bridge to a conductor contact which accesses an electrical circuit upon a substrate such as but not limited to a printed circuit substrate, a ceramic substrate and a semiconductor integrated circuit substrate; and (2) a method for using the electrical test apparatus to test the electrical circuit upon the substrate.
A second object of the present invention is to provide an electrical test apparatus and a method for using the electrical test apparatus in accord with the first object of the present invention, where there is avoided a contact resistance increase of the conductive probe tip which bridges to the conductor contact upon the substrate.
A third object of the present invention is to provide an electrical test apparatus and a method for using the electrical test apparatus in accord with the first object of the present invention and the second object of the present invention, where the electrical test apparatus and method for using the electrical test apparatus are readily manufacturable.
In accord with the objects of the present invention, there is provided by the present invention: (1) an electrical test apparatus employing a conductive probe tip to bridge to a conductor contact accessing an electrical circuit formed upon a substrate such as but not limited to a printed circuit substrate, a ceramic substrate and a semiconductor integrated circuit substrate; and (2) a method for using the electrical test apparatus when testing the electrical circuit formed upon the substrate. To practice the method of the present invention, there is first provided a substrate having an electrical circuit formed thereupon. The substrate also has a conductor contact formed thereupon which accesses the electrical circuit. The substrate is then positioned in a first position within an electrical test apparatus, where the electrical test apparatus has a conductive probe tip fixed within the electrical test apparatus. Within the first position, the conductive probe tip is positioned in the proximity of but not in contact with the conductor contact formed upon the substrate. The conductive probe tip is then repositioned to a second position with respect to the substrate such that the conductive probe tip contacts the conductor contact formed upon the substrate, while simultaneously purging the surface of the conductor contact with a purge gas stream. There is then obtained an electrical measurement of the electrical circuit within the substrate through the conductive probe tip in contact with the conductor contact while employing the electrical test apparatus. The method of the present invention also contemplates an electrical test apparatus through which the method of the present invention is practiced.
Through the present invention there is provided: (1) an electrical test apparatus; and (2) a method for using the electrical test apparatus, where the electrical test apparatus employs a conductive probe tip to bridge to a conductor contact which accesses an electrical circuit formed upon a substrate such as but not limited to a printed circuit substrate, a ceramic substrate and a semiconductor integrated circuit substrate, while avoiding a contact resistance increase of the conductive probe tip which bridges to the conductor contact formed upon the substrate. The electrical test apparatus and method of the present invention achieve the foregoing goals by employing a gas purge of the surface of the conductor contact simultaneous with repositioning the conductive probe tip from a first position in the proximity of but not contacting the conductor contact to a second position contacting the conductor contact.
The apparatus and method of the present invention are readily manufacturable. The apparatus and method of the present invention employ a gas purge upon the surface of a conductor contact formed upon a substrate, such as but not limited to a printed circuit substrate, a ceramic substrate and a semiconductor integrated circuit substrate, when a conductive probe tip is repositioned from a first position in the proximity of but not in contact with the conductor contact to a second position in contact with the conductor contact. Methods, materials and fabrications through which a purge gas flow may be introduced within an electrical test apparatus are, as illustrated within the preferred embodiment of the method and apparatus of the present invention, readily designed and constructed. Thus, the apparatus and method of the present invention are readily manufacturable.